1. Field of the Invention
The invention relates to a battery using a metal oxide enabling intercalation of alkali metal ion, as an active material for cathode, and a polymeric solid electrolyte as an electrolyte, the battery having improved discharge rate properties and stability at an elevated temperature.
2. Description of the Related Art
Recently, highly integrated and highly functional devices represented by very large scale integrated circuits (VLSIs) have been realized to a remarkable progress in microelectronics, especially techniques for the manufacture of semiconductor functional elements. By employing these devices for controlling systems in various pieces of equipment, rapid miniaturization of electrical equipment has been achieved, contributing to miniaturization and multifunctionalization of not only electrical equipment for various industries but also domestic electrical equipment.
Such electrical equipment is being directed toward cordless equipment which has an independent power supply device, and is operable while not relying on a commercial power supply. As a power supply, a battery is generally used, and among others, a so-called secondary cell which can be repeatedly charged for reuse is useful as a power supply for a portable information device such as a pocket telephone or hand-held personal computer because of the very low cost per one charge-discharge cycle. For the sake of miniaturization and lightening of the entire electronic equipment and operation for a longer time, development of a high performance battery having a smaller size and a higher capacity has been demanded.
As a battery suitable for miniaturization and lightening of the entire electronic equipment, attention has been paid to a so-called lithium secondary battery using a redox reaction of lithium ion. In recent years attention has been paid to a battery using, as an electrolyte which is an important component in a battery, a polymeric solid electrolyte. This is because the polymeric solid electrolyte has advantages in that it gives improved safety because of absence of problem of liquid leakage, which is a problem common to batteries using an electrolyte of solution type, and low flammability compared with prior solution-type batteries, and it makes it possible for the battery itself to have an improved processability and provide a battery having a thinner and freer shape because of excellent productibility of the polymeric solid electrolyte.
In a prior secondary battery using lithium, as set forth above, a system is used as a polymeric solid electrolyte, the system providing a larger ion electrical conductivity as a whole by adding an organic solvent having a higher polarization as an electrolyte component in a matrix of polymer material having a relatively high dielectric constant, such as polyethylene oxide, polyacrylonitrile, polyvinyl pyridine, polyvinyl chloride, polyvinyl alcohol, or a derivative obtained by introducing in the polymer material a reactive functional group such as acryloyl, vinyl or epoxy. This is because in a lithium secondary battery, the lithium ion as a carrier of electrical charge originates in a lithium salt such as lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, and lithium tetrafluorosulfonate, and the salt must sufficiently dissociate in an electrolyte, and the lithium ion must move in a sufficient rate to cancel polarization resulting from charging and discharging the battery.
As a solid electrolyte satisfying these conditions, a system can be used which is obtained by, for example, adding a non-aqueous organic solvent such as propylene carbonate to a polyethylene oxide, and further adding to the resultant mixture 1 to 3M of lithium hexafluorophosphate. Such a system has the advantage of providing a thin sheet-like battery, providing flexibility in the battery shape.
In the secondary battery system as referred to above, a powder of a metal oxide enabling intercalation of alkali metal ion, such as lithium cobaltate, lithium imanganate, or lithium nickelate, is used as the cathode. By contact with the cathode material of metal oxide, the polymeric solid electrolyte, as referred to above, becomes capable of transporting an alkali metal ion, resulting from charging and discharging, to the cathode.
The oxide materials used for a cathode have, in general, insulating properties. Accordingly, it is necessary that a redox current (electrons) resulting from alkali metal ions entering and leaving the cathode is transported to an external electrode in some way. Generally, a carbon-based conductive material having a high conductivity, such as acetylene black, is mixed with metal oxide powder for a cathode, to thereby lower the resistance of the cathode.
Contact of a metal oxide material with a carbon-based conductive material, which are both powdery or particulate, is point-contact, and relative location of the metal oxide material and carbon-based conductive material particles is changed while ions enter and leave the cathode. Consequently, an electrical resistance for transporting electrons from the cathode to the outside is apt to be spontaneously increased with the passage of time. Since the metal oxide particle has a surface of complicated configuration, it is not easy for, in particular, the carbon-based conductive material to come in contact with a deep site in the configuration. In addition, when a temperature of the battery is increased during the use thereof, an electrolyte component is expanded and changes in volume, to cause a change of location of the carbon-based conductive material in contact with the metal oxide and to cause the resistance to increase, so that there can be a case where discharge rate properties, particularly at an elevated temperature, are adversely affected.
Japanese Unexamined Patent Publication (Kokai) No. 8-236156 describes a battery having a cathode consisting of a conductive polymer or a transition metal oxide, or both. Only a polyaniline is used for the cathode in the examples, and no cathode using both of the conductive polymer and the transition metal oxide is shown.
Japanese Unexamined Patent Publication (Kokai) No. 6-271655 describes infiltrating a monomer forming a conductive polymer and a supporting electrolyte into an electrode of a porous body and forming a conductive polymer film on the entire surface inside the porous body electrode by electrolytic oxidation in the supporting electrolyte solution for the manufacture of an electrode used in a capacitor. However, this publication does not describe an electrode formed of metal oxide particles having, in part, on their surface, a polymer formed from a monomer.
Japanese Unexamined Patent Publication (Kokai) No. 10-27615 describes an electrode using a combination of a conductive polymer and a metal oxide, and indicates a polyaniline as the conductive polymer. In this reference, however, as is evident from metal oxide powder being added to a solution in which the polyaniline is dissolved in advance, the conductive polymer is not formed on the surface of the metal oxide powder, and is solvent-soluble.
It is an object of the invention to provide a battery which is excellent in discharge rate properties and high-temperature stability by use of an improved conductive material in contact with a metal oxide in a cathode.
The battery of the invention comprises an assembly of a cathode consisting mainly of a metal oxide enabling intercalation of an alkali metal ion and an electrically conductive material, a cathode collector in contact with the cathode, an anode of a material selected from the group consisting of an alkali metal, an alloy of an alkali metal and a metal of group II or III, and a carbon material capable of occluding alkali metal ion, an anode collector in contact with the anode, and a polymeric solid electrolyte interposed between the anode and the cathode, and an external packaging material sealing the assembly, wherein the electrically conductive material for the cathode is an electrically conductive polymer formed by polymerizing a monomer on the surface of particle of the metal oxide constituting the cathode.
The battery of the invention can be manufactured by a method of manufacturing a battery comprising preparing an assembly of a cathode consisting mainly of a metal oxide enabling intercalation of an alkali metal ion and an electrically conductive material, a cathode collector in contact with the cathode, an anode of a material selected from the group consisting of an alkali metal, an alloy of an alkali metal and a metal of group II or III, and a carbon material capable of occluding alkali metal ion, an anode collector in contact with the anode, and a polymeric solid electrolyte interposed between the anode and the cathode, and sealing the assembly by an external packaging material, the method comprising a step of making the electrically conductive material for the cathode by polymerizing a monomer on the surface of particle of the metal oxide constituting the cathode in a solution in which the monomer is dissolved.